The demand for a better communication quality and a higher transmission rate is brought forward by a future radio communication system, and a Multi-Input Multi-Output (MIMO) system with multiple antennas is extensively expected for its better channel capacity. A diversity technique is of a special importance among massive techniques which are researched and applied to the MIMO system. According to the diversity technique, a signal in a wireless network is transmitted or received via multiple antennas so as to prevent a system loss; here the system loss is caused by a deep channel fading from which the signal suffers. The application of the diversity technique is based on communication channels independent of each other. Generally speaking, two spatial channels provided by two antennas are considered to be independent of each other when the distance between the two antennas is greater than half of a wavelength adopted. For example, in wireless LAN series standards which are compatible with the Bluetooth technology, some standards may require that two antennas are more than 7 cm away from each other to ensure that the spatial channels provided by the antennas are independent of each other, while other standards may require that the two antennas are more than 2.8 m away from each other. It can be concluded that, since multiple antennas cannot be installed into sufficient space in a mobile terminal to ensure the independence of multiple communication channels due to the volume limitation of the mobile terminal, a receive diversity technique on the downlink and a transmitting diversity technique in uplink cannot be implemented.
In a conventional method, a virtual multiple antenna system consisting of a plurality of mobile terminals is capable of improving spectrum efficiency in the radio communication system. In the virtual multiple antenna system, a plurality of mobile terminals are regarded as one transmission device. In uplink, signals are transmitted to a base station which may isolate the signals from each mobile terminal by using decoding principles in a spatial multiplex technique.
For example, in uplink of the radio communication system, there are two mobile terminals and a base station, the two mobile terminals can be regarded as a transmission devices capable of transmitting signals to the base station with the antennas of the mobile terminals. The base station receives a data stream which is combined by two independent data streams. It can be seen that the virtual multiple antenna transmission device adopts a spatial multiplexing technique.
FIG. 1 is a schematic diagram illustrating a virtual multiple antenna system, in which Mobile Terminal 1 transmits signals to a base station via an antenna while Mobile Terminal 2 also transmits signals to the base station via its antenna. In such case, the whole uplink can be regarded as a virtual multiple antenna transmission system which equals a transmitter with two antennas capable of transmitting signals to the base station, and the base station receives two independent data streams.
However, the above scheme is just applicable to the transmission of uplink and unable to be used in the transmission of downlink, that is to say, the virtual multiple antenna system is just able to implement transmitting diversity but unable to implement receiving diversity.
In addition, the spatial multiplexing technique used in the scheme is unable to get a low system Bit Error Rate (BER) while improving spectrum efficiency. Since the signals received in the scheme are the combination of signals transmitted by a plurality of mobile terminals, the interference among the signals transmitted by a plurality of mobile terminals imposes on the virtual multiple antenna system a BER higher than the BER in a single antenna system. Even though the maximum likelihood (ML) criteria detection which is the most effective error detection method is adopted in the receiver, the BER of the virtual multiple antenna system is still relatively high.